1. Field of the Invention
The present invention relates generally to the art of cross-field microwave electron tubes for converting electron potential energy into high efficiency microwave energy. More specifically, the present invention relates to a radio frequency (RF) magnetron or microwave power tube that utilizes chemical vapor deposited (CVD) diamond as the cathode to increase the secondary electron emission of the magnetron during operation.
2. Discussion of the Prior Art
Most all vacuum electron tubes require a physical source of electrons which are typically provided by some method of electron emission. General electron emission can be analogized to the ionization of a free atom. Prior to ionization, the energy of electrons in an atom is lower than electrons at rest in a vacuum. In order to ionize the atom, energy must be supplied to the electrons in the atom. That is, the atom fails to spontaneously emit electrons unless the electrons are provided with energy greater than or equal to the electrons at rest in a vacuum. Energy can be provided by numerous means, such as by heat or irradiation with light. When sufficient energy is imparted to the atom, ionization occurs and the atom releases one or more electrons.
Several types of electron emission are known. Thermionic emission involves an electrically charged particle emitted by an incandescent substrate (as in a vacuum tube or incandescent light bulb.) Photoemission releases electrons from a material by means of energy supplied by incidence of radiation, especially light. Electron injection involves the emission of electrons from one solid to another. Field emission refers to the emission of electrons due to the application of an electric field to a cathode. Finally, secondary emission occurs by bombardment of a substance with charged particles such as electrons or ions.
A magnetron is one known type of microwave electron tube which generally utilizes the methods of thermionic emission and secondary emission to generate electrons. Magnetrons typically have a cylindrical symmetry. On the central axis is a hollow cylindrical cathode having pole pieces, such as magnets, disposed at each of its axial ends. The outer surface of the cathode carries electron-emitting materials, such as barium and strontium oxides. At a radius larger than the outer radius of the cathode is an annular anode.
In the operation of a conventional magnetron, a current is applied to the cathode which heats it to an elevated temperature in the vicinity of 1000xc2x0 C. The thermionic heat source provides the necessary energy to allow primary electrons to escape from the electron-emitting materials of the cathode. An electric field is applied radially inward from the anode while a magnetic field is generated from the opposed magnets in a direction perpendicular to the electric field. The magnetic field and electric field interact to produce a cross-field configuration that causes the emitted electrons to rotate azimuthally within the magnetron. Electrons with an optimum trajectory travel in a circular pattern and induce RF power in an outer cavity of the magnetron. Electrons with insufficient energy spiral back to the cathode and collide with the cathode""s surface, thereby producing secondary electrons. The secondary electrons are accelerated due to the crossed fields and become part of the electron cloud.
The secondary electron co-efficient is the number of secondary electrons that are produced due to a single electron impinging on the cathode surface. Tungsten, which is the material most commonly used as the cathode in known magnetron devices, has a secondary electron coefficient of less than 2 at the operation voltage. Once the magnetron reaches its operating level, much of the electron emission is sustained with secondary electron production. However, secondary electrons only make up approximately 60% of the overall electron emission in known magnetron devices. Thus, the cathode must be continuously heated in order to produce the remaining electrons needed for effective operation.
Magnetrons of the foregoing nature, which rely on thermionic emission for operation, have several shortcomings. First, the continuous use of an external source for generating a primary source of electrons is costly, especially in space communication applications. Second, electrons in such devices emerge from the cathode surface in all three Cartesian directions and at various angles to the surface normal causing a crossing of electron trajectories on the microscopic scale. As a result, the signal and the power generated have an abundance of electron noise which prevents the use of RF magnetrons in space communication applications. Third, the relatively high input power required for thermionic magnetrons makes their use in residential appliances, such as clothes dryers, rather costly. Finally, the necessity of heating thermionic cathodes limits the magnetron expected life, causes warm-up delays, and requires bulky ancillary equipment such as a peripheral cooling system.
U.S. Pat. No. 5,796,211 (the ""211 patent) discloses a traveling wave tube (TWT) having a cathode coated with ultrafine diamonds which is said to alleviate some of the above-identified problems. However, the ""211 patent is directed only to devices which rely on primary electron emission as opposed to primary and secondary electron emission. The ""211 patent neither discloses nor suggests the use of a diamond coated cathode in magnetron devices. In each of the devices disclosed in the ""211 patent, all of the electrons produced interact with an input signal which amplifies the electrons. There are no electrons directed back toward the cathode which produce secondary electrons. Accordingly, a primary electron producing source, such as an electric field, a heat source, etc., must be continuously applied to the cathode to generate a primary source of electrons. As noted above, the need for a continuously operating external source is quite costly.
Accordingly, a need exists to provide an RF magnetron device which overcomes the foregoing problems and others and which can sustain effective operation without an external electron generating source.
In accordance with one aspect of the present invention a radio frequency (RF) magnetron device for generating microwave power includes a cathode disposed within an electron tube. The cathode is at least partially formed from a diamond material. The diamond material is configured to emit electrons and sustain operation of the magnetron device via secondary electron emission and without assistance of a heating source. An anode is disposed concentrically around the cathode. An electron field is provided radially between the anode and the cathode. First and second oppositely charged pole pieces are operatively connected to the cathode for producing a magnetic field in a direction perpendicular to the electric field.
In accordance with another aspect of the present invention a secondary electron emitting device for a magnetron includes a cathode at least partially formed from a diamond material. The diamond material is configured to emit electrons and sustain operation of the magnetron via secondary electron emission and without assistance of a heating source.
In accordance with another aspect of the present invention a method for producing radio frequency power using a magnetron device includes coating a cathode with a diamond material. An anode is placed in a spaced relationship with the diamond coated cathode. An electric field is applied between the anode and the cathode. A magnetic field is applied perpendicular to the electric field. Primary electrons are emitted from the diamond coated cathode for initiating operation of the magnetron device. Secondary electrons are emitted from the cathode for sustaining operation of the magnetron device.
One advantage of the present invention is the provision of a magnetron device capable of sustaining operation without an external source for producing primary electrons, such as heat source, thereby significantly reducing the cost of operation.
Another advantage of the present invention is the provision of a magnetron device having a well defined electron emission which minimizes electronic noise.
Another advantage of the present invention is the provision of a magnetron device having an increased operating life.
Another advantage of the present invention is the provision of a magnetron device which eliminates warm-up delays.
Another advantage of the present invention is the provision of a magnetron device which minimizes the need for ancillary equipment.
Yet another advantage of the present invention is the provision of a magnetron device capable of emitting electrons upon application of a relatively low level of voltage.
Still another advantage of the present invention is the provision of a magnetron device having increased efficiency and output due to the high secondary electron coefficient of the diamond coated cathode.
Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed specification.